Never got the chance to try the manticore but I have no doubt it was real.

Now I thought to myself oh it was probably just a fluke that the AI is casting real creatures so I tried it again on a different map and sure enough AI casts real creatures (No illusion) on low casting chance and they work every time

Not sure if this is a bug or AI is designed to cast real creatures in the face of certain tabbos

I played two games with Hard and Easy A.I. In both games I disbelieved an illusion and in both games the A.I. failed to cast a spell.

I thought maybe the rewards were different from Gryth's since I had Forge Crystals set. I played a third game on A.I. Easy with High-Casting Chance taboo and rewards set to none. I disbelieved one spell and A.I. failed to cast a spell.

Except in certain tutorials where illusions are disabled, the AI casts illusions based on a few factors, but mostly to do with how "believable" the illusion will be. I.e., you'll tend not to see the AI cast a fake turn-1 hydra (but, I believe it's possible:). Over a small number of games, you can get some surprising results (all fake, or all real creatures).
I'm not sure how the AI will cope with taboos that force it to cast "difficult" creatures - it may burn heavily to make the casts happen.

The AI will always disbelieve "sure" illusions - if it kills one of a group of multi-cast creatures (e.g., rats, zombies or various megaspells) and one is an illusion, it will disbelieve the rest as soon as it can.

Otherwise, the AI has a somewhat complicated set of "disbelieve" rules, that I'm not going to disclose;p